1. Field of the Invention
This invention relates to a method of forming on a substrate a thin film such as a functional film used as a reference electrode of an ion sensor, and apparatus for forming a thin film suitable for carrying out the method and a biosensor and chemical sensor such as an ISFET (ion sensitive field effect transistor) sensor having a thin film formed by the apparatus.
2. Description of the Prior Art
Ion sensors or the like generally use reference electrodes for reference potential generation. In the manufacture of this reference electrode, there have been many attempts of manufacturing hydrophobic organic thin films consisting of hydrophobic fluororesins, particularly polytetrafluoroethylene (available under a trademark "Teflon") by a dry process such as a deposition process or a sputtering process.
However, in a method of forming a thin film of a reference electrode by the prior art sputtering process or the like, it is very difficult to control film composition, directivity of film and polymerization degrees, and it has been impossible to form an organic thin film as designed on a substrate. In the sputtering process, heat is stored in a target by an accelerated beam, thus causing thermal damage to the target. This technical field is only in an initial stage, and it can be said that the selection of the kind of organic thin film and corresponding dry process determines the quality of the film and characteristics of the intended functional film.
As the above apparatus for manufacturing a thin film, used in the field of manufacture of semiconductors, there are known various CVD (chemical vapor deposition) apparatus for instance, thermal CVD, reduced pressure CVD, light CVD and plasma CVD apparatuses, sputtering apparatus and ICB (ion cluster beam) apparatus. In many of these apparatuses, film is formed in vacuum and under a somewhat high temperature condition.
Recently, there have been extensive studies with an aim of making use of organic thin films for improving the function of electronic devices. It is thought that organic thin films can find extensive applications for passivation films and sealing materials because it is possible to obtain films which have good machining properties and are chemically stable.
However, with such prior art apparatuses for dry state manufacturing of thin films it is necessary to form films at high temperatures. Therefore, it is very difficult to form organic substances capable of ready decomposition, particularly thin films of polymers. Further, it is difficult to form multi-layer bodies, laminations and mixtures of thin films.
By using the RF sputtering process or ion beam process it is possible to form multi-layer bodies and mixtures of thin films. These methods, however, lead to great damage with organic substances due to ion bombardment, and there is a high probability of fragmentation of molecules. Besides, there is a trend of introduction of ions. Further, it is impossible to form an insulating film.
As methods of forming thin film in a wet state, there are a LB (Langmuir Blodgett) process and an electrolytic process. In these processes, thin films can be formed at room temperature. However, substances which can be used for film formation are limited, and thus it is impossible to obtain a perfect insulating film.
Further, regarding a sensor using a substrate with a thin film as electrode, ion sensors utilizing MOSFETs capable of fine machining using semiconductor manufacture technology, i.e., ISFET sensors, have been developed as solid-state very small sensors, with reported sizes less than 10 microns.
While the active electrode of the sensor is capable of miniaturization in this way, miniaturization of the whole sensor requires miniaturization of the reference electrodes.
With the prior art technique, however, it is difficult to obtain miniaturized solid reference electrodes. In order to solve these problems, reference electrodes with polystyrene thin film and hydrophobic organic polymer film on a gate insulating film are disclosed in Japanese Patent Disclosures 58-103658 and 58-34352.
While various reference electrodes have been developed for miniaturization and solidification, these electrodes can readily respond to pH (hydrogen ion concentration). In addition, adsorption of and permeation to interfering substances are possible. Therefore, these reference electrodes have inferior potential stability, and difficulties are involved in their use.
As further sensors, there are enzyme sensors for measuring the substrate concentration using enzyme electrodes by an amperometric process, and particularly enzyme sensors for directly measuring enzymic reaction from oxidation/reduction reaction of electron movement media (mediators).
These kinds of enzyme sensors are mainly used for clinical chemical analysis, and those for glucose, urea and neutral and phosphoric fat as the subject of measurement are in practical use. For instance, an enzymic reaction in the case of .beta.-D-glucose (grape sugar) as the subject of measurement is expressed as ##STR1##
More specifically, B-D-glucose consumes O.sub.2 with an action of .beta.-D-glucose (GOX) to generate an organic acid (gluconolactone) and hydrogen peroxide (H.sub.2 O.sub.2). Therefore, glucose concentration can be measured from the amount of generated hydrogen peroxide and gluconolactons or amount of consumed oxygen.
Heretofore, for measuring the glucose concentration according to the amount of generated hydrogen peroxide a method has been adopted, in which the generated hydrogen peroxide is oxidized with a metal electrode, and the oxidizing current is measured. Alternatively, the generated hydrogen peroxide is reduced, and the reducing current is measured. However, these oxidizing and reducing currents are subject to the influence of oxygen. Further, a prior art detection electrode is liable to be influenced by changes in the surface state. Further, the principle of measurement by electrochemical means uses a sensor structure, which comprises an electrode substrate, liquid, enzyme fixed film and liquid under test. Therefore, miniaturization of the sensor is difficult due to liquid present between the electrode and the film.
As other methods for measuring the glucose concentration, there are
(1) one, in which hydrogen peroxide is decomposed by catalase, and the amount of oxygen is measured, and
(2) one, in which iodide ions are oxidized in the presence of enzyme (peroxidase) and inorganic catalyst (molybdenum), and the amount of iodine is measured by causing the following reaction, thereby indirectly measuring the amount of hydrogen peroxide. ##STR2##
In the above way, when measuring the glucose concentration according to the amount of generated hydrogen peroxide, the amount of consumed oxygen or amount of generated iodine is measured, and the amount of generated hydrogen peroxide is determined indirectly from the measured amount.
However, such method of measurement through two reaction stager requires a separate electrode for measuring oxygen or iodine in addition to an enzymic electrode for decomposing glucose or the like to hydrogen peroxide. In addition, the measurement is very complicated and takes a long time. Meanwhile, with the prior art electrochemical method, there are problems of contamination of the liquid under test and difficulties of miniaturization because the sensor contains inner liquid.